Stimulants as Specific Inducers of Dopamine-Independent s Agonist Self-Administration in Ratss

A previous study showed that cocaine self-administration induced dopamine-independent reinforcing effects of s agonists mediated by their selective actions at s1 receptors (s1Rs), which are intracellularly mobile chaperone proteins implicated in abuserelated effects of stimulants. The present study assessed whether the induction was specific to self-administration of cocaine. Rats were trained to self-administer the dopamine releaser, d-methamphetamine (0.01–0.32 mg/kg per injection), the m-opioid receptor agonist, heroin (0.001–0.032 mg/kg per injection), and the noncompetitive N-methyl-D-aspartate receptor/channel antagonist ketamine (0.032–1.0 mg/kg per injection). As with cocaine, self-administration of d-methamphetamine induced reinforcing effects of the selective s1R agonists PRE-084 [2-(4morpholinethyl)1-phenylcyclohexanecarboxylate hydrochloride] and (1)-pentazocine (0.032–1.0 mg/kg per injection, each). In contrast, neither self-administration of heroin nor ketamine induced PRE-084 or (1)-pentazocine (0.032–10 mg/kg per injection, each) self-administration. Although the s1R agonists did not maintain responding in subjects with histories of heroin or ketamine self-administration, substitution for those drugs was obtained with appropriate agonists (e.g., remifentanil, 0.1–3.2 mg/kg per injection, for heroin and (5S,10R)-(1)-5-methyl-10,11dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine ((1)-MK 801; dizocilpine), 0.32–10.0 mg/kg per injection, for ketamine). The sR antagonist N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(1pyrrolidinyl)ethylamine dihydrobromide (BD 1008; 1.0–10 mg/kg) dose-dependently blocked PRE-084 self-administration but was inactive against d-methamphetamine, heroin, and ketamine. In contrast, PRE-084 self-administration was affected neither by the dopamine receptor antagonist (1)-butaclamol (10–100 mg/kg) nor by the opioid antagonist (2)-naltrexone (1.0–10 mg/kg), whereas these antagonists were active against d-methamphetamine and heroin self-administration, respectively. The results indicate that experience specifically with indirect-acting dopamine agonists induces reinforcing effects of previously inactive s1R agonists. It is further suggested that induced s1R reinforcing mechanisms may play an essential role in treatment-resistant stimulant abuse, suggesting new approaches for the development of effective medications for its treatment.